Several lines of evidence suggest the existence of two functionally separate visual systems. One is the preattentive system; it can quickly, effortlessly, and in parallel detect differences in a variety of conspicuous features. The other system involves focal attention; it is used when difficult or complex discriminations must be made. It proceeds serially and effortfully. One major purpose of the present research is to clarify how preattentive and focal attentive processes cooperate in the performance of tasks, and, in particular, how preattentive processes may guide attentive ones. A second major purpose of this research is to explore the spatial properties of attention. This work will consider the nature of the coordinate system(s) that may underline attentional processes. What determines whether processing is retinocentric, environmentally based, object based, or object centered? One set of experiments will consider the factors that permit a subject to search selectively through large, complex displays. Some previous studies have found that subjects can search through a complicated array without being distracted by irrelevant information, whereas other studies have shown a variety of distraction effects (such as attention being capture by conspicuous irrelevant stimuli). This research will attempt to ascertain the principles that determine the efficiency of search. Other experiments examine the medium in which attention operates. Imagine, for example, that with the eyes held steady a cue directs attention to a particular location. Shortly thereafter a small, faint, target is presented. The ability to detect such a target is greater when it appears near the location of the cue than when it appears far from the cue. The question is, What spatial representation mediates the changes in performance? Is performance determined by the separation on the retina between the cue and the target, or by their separation in perceived three-dimensional space? Suppose the cue falls on part of an object. Does the structure of the object determine how attention is allocated? These and similar questions will be addressed.